The present invention relates to a power converter for converting a DC power supply voltage into three phase output voltages having an improved voltage utilization factor.
In prior art methods for improving the voltage utilization factor, the same auxiliary voltage value is added to each phase of three phase output voltage commands to obtain each PWM signal, and space vectors are calculated from switching conditions in the power converters. As used herein, improving the voltage utilization factor refers to any method by which a relatively large line-to-line voltage in a sinusoidal wave can be obtained, although its phase voltage produced is low.
Known processes of the above type are described, for example, in JP-A Laid-Open No. 3-107373, U.S. Pat. No. 4,321,663 and JP-A Laid-Open No. 2-285967. JP-A Laid Open No. 3-107373 describes a method to improve the voltage utilization factor (efficient modulation method of output voltage) through the steps of identifying the one of three phase output voltage commands which has an intermediate magnitude of voltage thereamong, and adding to each phase a voltage which is obtained by multiplying the one thus identified by 1/2. U.S. Pat. No. 4,321,663 and JP-A Laid Open No. 2-285967 are based on the same concept to obtain PWM signals in an identical manner.
A typical example of the known methods of producing PWM signals utilizing the space vectors, is described in U.S. Pat. No. 4,480,301. Taking into account eight switching states (seven with respect to voltage space vectors) available for the three phase power converter, this prior art method combines these eight switching states for respective time duration in order to obtain desirable voltage vector commands. By this method, the voltage utilization factor can be improved as well.
One method for controlling three phase AC motors is generally known as the vector control method, in which voltage command vectors are calculated in a rotating coordinate system. This method is particularly advantageous for a high output frequency of the power converter. Further, when the output frequency of the power converter increases, the speed of the motor as well as the voltage to be applied also increase, so that a substantial improvement in the voltage utilization factor becomes necessary.
Under such circumstances, according to the above prior art processes, after calculation of respective three phase voltage commands utilizing the voltage command vector on the rotating coordinate system obtained by vector control, the auxiliary voltage to be added to each phase must be calculated from these phase voltage commands. Then, it is necessary to add the auxiliary voltage thus obtained to each phase, and to process PWM signal generation. In this case, since extensive arithmetic operation must be carried out in order to improve the voltage utilization factor, when a conventional microprocessor is used, substantial processing time is required.
Further, in the prior art method in the latter case described above, it is necessary to select a switching state, calculate its duration, and output the same for each cycle of the PWM signal (for example, at every 100 .mu.s for switching frequency of 10 kHz). This method is advantageous in that it can generate PWM signals directly. However, it also suffers from the disadvantages that the process cycle for calculation required is short, and that substantial processing time is required to judge conditions and calculate the duration. Thus, use of a high performance microprocessor becomes essential.